Automatic tone control system



P. F. G. HOLST 2,078,762

..AUTOMATIC TONE CONTROL SYSTEM Filed Jan. 2; 1936 April 27, 1937.

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Patented Apr. 27, 1937 PATENT OFFIOE AUTOMATIC TONE CONTROL SYSTEM Poul F. G. Holst, Oaklyn, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 2, 1936, Serial No. 57,121

14 Claims.

The present invention relates to automatic tone control system for radio receivers] and the like adapted to be actuated in response to carrier wave variation or signal strength to varythe audio frequency response characteristics thereof. In accordance with the usual arrangement, the present device provides for controllingthe fidelity of response with variations in signal strength or carrier wave amplitude.

The present invention has for itsobiect to provide an improved-tone control systemresponsive in variations in carrier wave amplitude or signal strength and including a voltage variable reactance path in connection with an audio frequency circuit to control the audio frequency response characteristic of the system, particularly at the high frequency end of the audio frequency response range thereof. The action of the automatic tone control is preferably'made to increase the high frequency response with signal strength or carrier wave amplitude.

Itis a further object of the present invention to provide animproved automaticv tone control system of the above character which operates on the principle that an apparent capacity may be changed through the variation of the gain in an amplifier stage, the amplifier stage and circuit in connection therewith providing amplification after the control action has taken place, whereby signal distortion may materially be reduced.

It is a further object of'the present invention to provide an automatic tone control system for radio receiving apparatus and the like which is readily applicable to the usual audio frequency amplifier means provided in such apparatus with-' out material change in the construction, thereof and without introducing appreciable distortion as a result of control operation.

Itis also a further object of the present invention to provide an improved automatic tone control system which may utilize a voltage ,variable reactance path including a capacitance device and an electric discharge amplifier device wherein the amplifier device operates as a voltage variable feed back means as distinguished from the usual variable impedance means, heretofore known and used.

The invention will, however, be better understood from the following description when con-, sidered in connection with the accompanying drawing and its scope will be pointed out in the appended claims.

In the drawing, Figure 1 is a circuit diagram of an audio frequency amplifier circuit provided with automatic tone control means embodying the invention. and

Fig. 2 is a similar circuit diagram of a portion of an audio frequency amplifier provided with an automatic tone control system embodying the in- 5 vention, as a-modification of the circuit shown in. Fig. 1.

Referring to Fig. 1, 5 is an audio frequency amplifier electric discharge device or tube for amplifying audio frequency signals supplied thereto .between ground 6 and an audio frequency input lead I forming the two sides of an audio frequency amplifying channel. The amplifier tube 5 is provided with a grid input lead 8 connected with the lead I through a series inductor 9 and to ground 6 through a shunt resistor or impedance element Ill. Self biasing potential is supplied to the grid of the amplifier from a self bias resistor H. l

'The audio frequency output channel for the amplifier is indicated at l2 and includes an output coupling impedance l3 for the device 5 and a coupling capacitor M in connection with an audio frequency output lead l5 between which and ground the amplified signals are supplied for further utilization by any suitable means (not shown).

An amplifier 20 is connected with the amplifying channel to receive signals therefrom through the shunt impedance ill from a low potential tap point 22 thereon through the intermediary of variable gain amplifier stage 23 including an electricdischarge amplifier device 24, the latter being preferably of the variable mu pentode type, as shown, comprising an output anode 25, a cathode 28, a control'grid 21 and screen and suppressor grids 28 and 29, respectively. The control grid 21 is coupled to the voltage tap point 22 through a coupling capacitor 30 and receives controlling biasing potentials through a grid resistor 3| from a bias potential supply lead 32, which may be connected with automatic volume control means or other suitable controlling voltage source, as indicated.

' The output anode 25 is coupled through a cou- 5 pling impedance 35 and a coupling capacitor 38 with an input grid circuit 31 of the amplifier 20, in which the grids of the amplifier devices 38 receive fixed biasing potentials from suitable means, such as self' bias resistors 39. The gain of the amplifier 20 is therefore constant as is desired in the present circuit.

With the arrangement shown, a control point I! on the audio frequency amplifying channel is preceded by a series inductor 9 and is connected off point.

with a feed back capacitor I 6 and lead] I 8 in series with a voltage variable gain feed back means 23, and a constant gain amplifier 20, the former being connected to the point H through a voltage step down device provided by the tapped impedance Ill. The output coupling device 40 of the constant gain amplifier 20 is connected with the feed back lead i8.

With this arrangement, a relatively small or fractional portion of the audio frequency available at the control point I! is utilized for control purposes from the tap 22. This portion of the audio frequency voltage can be made so low that the distortion created by passing it through the voltage variable device 23 is negligible for all control voltages applied through the lead 32. Otherwise the distortion would be amplified and applied back on the audio frequency circuit.-

Tliis relatively small portion of the signal voltage may be made to control the tone characteristic of the amplifier channel by applying it to the voltage variable gain stage followed by the fixed gain voltage amplifier, and feeding back the controlled and amplified voltage to the control point which is at a higher audio frequency potential than the initial supply point 22.

This action causes the current flow through the capacitor l6 to vary with gain in thestage 23 at a higher rate depending on the gain thereby to permit tuning with the inductor over a wider range as a low pass filter having a variable peak which is moved in the direction to increase the high frequency response withincreased carrier wave or signal strength. In other words, the effect of the capacitor It in connection with the series inductor 9 may be controlled by the gain of the amplifier device inserted between 22 and I8 and, through it, signals derived from the audio frequency channel may be fed back to the point I! through the capacitor l6, which if done in the proper phase will cause the capacitor I 6, to function as if located at C, in shunt to the audio frequency channel. The value of C is dependent on the gain of the audio amplifier inserted between 22 and 18. The variable mu type tube 24 has been inserted in order to provide this variation in gain. It is to be noted that the proper phase relation must be maintained in order to obtain the desired results. A phase reversal between the point 22 and the output end of the amplifier 20 to the lead I8 is preferably approximately 180 or 540, etc. v

The capacitor 30 may be varied as well as other capacitors 36, 4| and 42 in the output circuit of the variable gain amplifier or otherwise along the amplifying circuit, may be varied to control the phase of the feed back potential and the response characteristic at the filter cut Likewise the contact 22 is variable to control the amplitude of the feed back voltage manually and independently ofthe automatic control.

It has been found that with this system, the

amplifier device 20 and 23, in series with the shunt capacitor l6, operates as a source 'of audio frequency voltage and as a voltage-variablegain, feed back device rather than as a variable impedance device, so that a voltage variable reactance path predominately capacitive in char- 26 in. the control amplifier stage. This gain varies the feed back through the circuit I8. choosing the correct capacity IS, the latter may be made to act as a variable capacitor connected as shown at C to tune with theinductor 9. This system has the advantage that only a small portion of the voltage to be regulated is impressed upon the grid of the control stage 23. Further,

additional amplification is provided after the regulation has taken place to restore the loss in gain occasioned by the reduction in voltage at the potentiometer tap point 22. Therefore, the voltage impressed upon the control stage 23 may be kept relatively low, thereby making it possible to' reduce the distortion to any desired level. The additional amplification provided by the amplifier 20 can be arranged to produce very low distortion since it is operated at constant bias and may be arranged to give the desired amplification and phase shift.

Referring now to Fig. 2, in which the same reference numerals are used to designate like circuits and elements as in Fig. 1, the control amplifier device 23 interposed between the control point 22,on the impedance l0 and the fixed bias constant gain amplifier 20 is in the present circuit, a combination device 50 having a cathode 5|, an inner control grid 52, an outer control grid 53, an output anode 54, a suppressor grid 55 and a screen 56 for the outer control grid. Signals from the tap point 22 are applied to the inner control grid 52 through a. direct connection lead 51, while an automatic volume control or other control voltage is supplied from the lead 32 to the outer control grid 53. The output anode 54 is coupled through the coupling impedance 35 and the coupling capacitor 36 to the amplifier 20. The voltage output is supplied to the point l1 through the lead l8 and the capacitor It as in the previously described embodiment.

The device 50 may be of the type known as the RCA 6L! and has the advantage that the controlling potential is applied to a separate grid from the signal potential, and more effective control of the amplification is obtained. Furthermore the control circuit is effectively isolated from the signal circuit.

In this modification, as before, a variable bias and variable gain amplifier is connected to the signal channel through a potential reducing means and the. signals therefrom are further amplified by a fixed bias amplifier which serves to control a shunt capacitor in connection with a series inductor whereby a.- variable cut-ofi filter isv provided in the audio frequency channel and is controlled by controlling the gain of the amplifier. This circuit arrangement is' otherwise the same as the preceding example and no further description is believed to be necessary.

It will be noted that the contact 22 is variable as well as the capacity of the capacitors in the amplifier at 36 and 58 for example to vary the phase shift as in the preceding example.

Other modifications of the invention may include other suitable amplifying means between the low voltage tap point 22 and the controlling capacitor l6. However, in any case, the system should be arranged to control the amplitude and phase of the voltage supplied to the controlling amplifier and tooperate the shunt capacitor and series inductance as a voltage variable reactance path in the audio frequency circuit controlled by an electric discharge amplifier as a voltage variable feed back device therein. The effect of the capacity element in conjunction with the inductor element of the filter is amplified, resulting in a wide range of control of the tone with small change in the control voltage and low distortion.

I claim as my invention: 1. A tone control system for an audio irequency amplifier, comprising in combination, a series inductor. a shunt potentiometer device, a variable gain amplifier coupled to a low signal potential point on said potentiometer device, a capacitor connected with said inductor, and means for impressing amplified signals from said first named amplifier on said capacitor thereby to vary theefl'ective capacity in circuit with said inductor, whereby a low pass variable filter is provided.

2. A tone control system for an audio frequency amplifier, comprising in combination, a series inductor, a shunt potentiometer device, a variable gain amplifier coupled to a low signal potential point on said potentiometer device, a capacitor connected with said inductor, -and means for impressing amplified signals from said amplifier on said capacitor thereby to vary the effective capacity in circuit with said inductor, whereby a low pass variable filter is provided, means in circuit with said amplifier for varying the phase shift of signals transmitted therethrough, and means for applying a controlling potential to said amplifier thereby to vary the gain through a predetermined range.

, 3. In an audio frequency amplifier, the combination with an audio frequency signal amplifying circuit, of means providing a variable low pass filter therein, including a series inductor in'said circuit and a potentiometer device connected in shunt to said circuit at' a common point, a variable gain amplifier connected to a low signal potential tap point on said potentiometer to receive a relatively small portion of the signal potential across said circuit, a constant gain amplifier coupled to said first named amplifier, a tuning capacitor for said inductor connected with said first named point on the signal circuit whereby potentials are impressed upon said capacitor, and means for impressing opposing-potentials on said capacitor from said amplifier.

'4. In an audio frequency amplifier, the combination with an audio frequency signal ampliiying circuit, of means providing a variable low pass filter therein including a series inductor in said circuit and a potentiometer device connected in shunt to said circuit at a common point, a variable gain amplifier connected to a low signal potential tap point on said potentiometer to receive a relatively small portion of the signal po- I tential across said circuit, a constant gain am- 6 plifier coupled to said first named amplifier, a

tuning capacitor for said inductor connected with said first named point on the signal circult whereby potentials are impressed upon said I capacitor, means for impressing opposing po- 65 tentials on said capacitor from said amplifier, whereby the effective capacity provided by said capacitor is responsive to variations in the gain of said first named amplifier, and means for applying automatic volume control potentials to said first named amplifier to vary the gain thereof in response to variations in carrier wave amplitude.

5. In a tone control system, a variable band pass filter circuit including a capacity element 75 provided by a capacitor, a shunt potentiometer for said circuit connected to a common point thereon with the capacitor, variable gain control amplifier means connected with a low voltage tap on said potentiometer to receive audio signals therefrom, a constant gain amplifier coupled to said first named amplifier, and an audio frequency output circuit for the last named amplifier coupled to said common point through said capacitor.

6. In an audio frequency amplifier, the combination with an audio frequency signal amplifying circuit, of means providing a variable low pass filter therein including a series inductor in said circuit and a potentiometer device connected in shunt to said circuit at a common point, a variable gain amplifier connected to a low signal potential tap point on said potentiometer to receive a relatively small portion of the signal potential across said circuit, a constant gain amplifier coupled to said first named amplifier. a tuning capacitor for said inductor connected with said first named point on the signal circuit, whereby signals are impressed upon said capacitor, and means for impressing signals in opposition thereto from said amplifier through said capacitor whereby the effective capacity provided by said capacitor is responsive to variations in the gain of said first named amplifier.

7. An automatic tone control system for audio frequency amplifiers and the like, including a series inductance and a shunt potentiometer device connected to a common audio frequency signal voltage point, and a voltage variable feed back amplifier device coupled to said capacitor providing therewith a voltage variable reactance path across the audio frequency circuit at said point, and an automatic volume control circuit connected with said first named amplifier device to control the gain therein.

8. In an audio frequency amplifier comprising an electric discharge amplifier device having an input circuit, the combination of a potentiometer device in said input circuit, a series reactance through which audio frequency signals are applied through said potentiometer device, an electric discharge device coupled to said potentiometer device at a low potential tap point thereon, means for controlling the amplification through said device in response to signal variations, and means providing a voltage variable reactance path across said input circuit coupled to said amplifier to be controlled thereby and including a capacitance device and a voltage variable feed back device in series, said last named device being coupled to said amplifier for receiving amplified controlling audio frequency signals therefrom.

9. In an audio frequency amplifier circuit, means providing a ariable cut on low pass filter including series inductance and shunt capacity elements, said elements being connected with a common signal voltage point in said circuit, a voltage-variable-gain feed-back device in series with said capacitor, and means for applying audio frequency signals to said device including a shunt impedance device having 'a low potential tap point thereon for said device.

10. An automatic tone control system for audio frequency amplifiers and the like including series inductance and shunt capacity elements and a shunt potentiometer device connectedto a common signal voltage point, an amplifier coupled between said capacity element and said potentiometer device at a low potential tap thereon, said amplifier having a variable gain control gain thereof. a fixed gain amplifier coupled to said first named amplifier and having an audio frequency output circuit and a capacitor connected between said output circuit and the inductive element of said filter, thereby to provide a variable capacity element in said filter.

12. The combination with an audio frequency amplifier, of a tone control circuit therefor including inductive and capacitive low pass filter elements, means for deriving a portion of the audio frequency signal voltage in said amplifier,

a variable gain electric discharge amplifier for amplifying said portion of the audio frequency signal voltage, means for applying a variable controlling potential to said amplifier to varythe gain thereof, a fixed gain amplifier coupled to said first named amplifier and having an audio frequency output circuit and a capacitor connected between said output circuit and the inluctive element of said filter, thereby to provide a variable capacity element in said filter, said va- I riable gain amplifier including a signal grid and a control grid whereby the signal potentials and controlling potentials thereon may be applied through differing circuits.

13. The combination with an audio frequency amplifier, of a tone control circuit therefor including inductive and capacitive low pass filter elements, means for deriving a portion of the audio frequency signal voltage in said amplifier, a variable gain electric discharge amplifier for amplifying said portion of the audio frequency signal voltage, means for applying a variable controlling potential to said amplifier to vary the gain thereof, a fixed gain amplifier coupled to said first named amplifier and having an audio frequency output circuit and a capacitor connected between said output circuit and the inductive element of said filter, thereby to provide a variable capacity element in said filter, a potentiometer device having a variable tap for applying signal potentials to said first named amplifier and a variable capacitor between said tap and said amplifier to control the phase shift of signals applied to said amplifier.

14. In a radio receiving system having an audio frequency amplifier channel and automatic volume control means, the combination of an electric discharge amplifier stage controllable by said automatic volume control means, potentiometer means for coupling said stage to said audio frequency circuit, a capacitor connected to receive the output of said amplifier. an inductor tunable by said capacitor in said audio frequency circuit, thereby to provide a voltage variable reactance path predominately capacitive in character across said audio frequency circuit.

POUL F. G. HOLST. 

